Question

2,31 km=........dm ..

Answer 1

1 m = 10 dm
1 dam = 100 dm
1 hm = 1000 dm

1 km .......... 10000 dm    
2,31 km .......... x dm

$\Rightarrow \text{x} = \dfrac{2,31\cdot 10000}{1} \Rightarrow \text{x} = 23100$

=> 2,31km = 23100 dm

Answer 2

1 km = 10000 dm
2.31 km = 23100 dm